The field of the invention is systems and method for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for designing parallel transmission (“pTx”) radio frequency (“RF”) pulses for use in multislice MRI applications.
Parallel transmission RF pulse design methods have been proposed that allow uniform excitation of spins across the field of view and excitation of specific patterns “burnt” into the patient. Electromagnetic (“EM”) simulations of such pulses have shown that the interference of electric fields created by every channel deposit a large amount of energy, as measured by the specific absorption rate (“SAR” expressed in W/kg), at focal locations in the body. When not controlling local SAR explicitly in the pTx pulse design process (i.e. when controlling global SAR or pulse power), local SAR in pTx is often found to be 5 to 10 times greater than in single channel excitations. Another difficulty is that, in contrast to single channel excitation, the ratio of local to global SAR is not constant and depends on the pTx pulse being played. Therefore, both global and local SAR have to be monitored explicitly in pTx experiments.
Explicit control of local SAR in pTx is difficult because, in theory, it requires monitoring SAR at hundreds of thousands of locations in the body during the pulse design process and the actual scan. A pulse design algorithm that explicitly incorporates local SAR, global SAR, and peak and average power constraints has been proposed by D. O. Brunner and K. P. Pruessmann in “Optimal design of multiple channel RF pulses under strict power and SAR constraints,” Magnetic Resonance in Medicine, 2010; 63(5): 1280-1291; however, this study only showed a proof of concept by controlling SAR at a few locations in the body.
J. Lee, et al., proposed an approach that controls local SAR at a limited number of “virtual observation points” (“VOPs”), as described in “Local SAR in parallel transmission pulse design,” Magnetic Resonance in Medicine, 2012; 67(6):1566-78. In this method, fast computational control of local SAR in the entire body is achieved by using a reduced number of SAR constraints. This approach is not optimal, however, because it is based on an approximation of local SAR as a linear combination of the SAR values associated to all VOPs. In other words, it is possible to find pulses that have a better local SAR versus excitation fidelity than those found by this algorithm. Another limitation of this method is that it is not capable of incorporating other constraints than local SAR (e.g. global SAR and/or pulse power).
Another class of approaches for reducing local SAR at constant excitation error includes using distinct RF pulses instead of only one RF pulse, as is done conventionally. Such RF pulses can be designed so that they have cancelling SAR hotspot locations and, therefore, an overall six minutes average local SAR that is significantly lower than the local SAR of individual pulses. This is motivated by the fact that the FDA regulates only the six minutes average of local and global SAR, not instantaneous SAR. This type of local SAR reduction technique can be referred to as “SAR hopping” because it allows SAR hotspots to “hop” from pulse to pulse. A limitation of the previous approaches that make use of SAR hopping is that they do not control local SAR explicitly in the entire body, but only at a few locations. Pulses designed using this technique are, therefore, not optimal. A common problem with these approaches is that they use different pulses to excite a single slice of k-space. Since different pulses cannot have exactly the same excitation profiles, this results in ghosting and inaccurate image contrast.
It would therefore be desirable to provide a system and method for designing pTx RF pulses for multislice imaging applications that overcome the drawbacks of currently available methods, including suboptimal design performance and the inducement of ghosting artifacts.